Dehydration synthesis is the process of making a larger molecule from smaller building blocks by removing two hydrogen atoms and one oxygen atom -- the chemical components of water. Lipids are fats; in the body, common lipids include triglycerides -- dietary and storage fats -- and phospholipids, which form cell membranes. You produce your large biological lipids through dehydration synthesis.
As a class of chemical transformations, dehydration synthesis reactions are incredibly common, particularly in biochemistry. Your body has a wide variety of metabolic processes that involve building larger molecules out of smaller precursor molecules, many of which proceed through dehydration synthesis. During a dehydration synthesis, two smaller molecules collectively lose two hydrogen atoms and an oxygen atom. In the process, the smaller molecules form new bonds to one another, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Triglycerides are an important class of lipids, or fats. These are the types of lipids that you take in when you consume fat in your food; they're also the lipids that you store in adipose, or fat, tissue. Triglycerides consist of a chemical backbone made up of a glycerol molecule, which is similar in structure to alcohol. The triglyceride also contains three long chains of carbon and hydrogen, each of which has two oxygen atoms connected to one end. These long molecules are called fatty acids. When a plant or animal produces triglyceride, it connects each of the fatty acids to the glycerol backbone through dehydration synthesis.
Another important class of lipids in your body are the phospholipids. These are the major components of cell membranes, explains Dr. Lauralee Sherwood in her book "Human Physiology." Like triglycerides, they consist of a glycerol backbone. They also contain two fatty acids and a small molecule called a phosphate. Producing a phospholipid involves connecting each of the fatty acids and the phosphate group to the glycerol backbone through dehydration synthesis.
The reverse of the dehydration synthesis reaction is called hydrolysis, which literally means "splitting with water." When you break down triglycerides from food, you have to separate two of the fatty acids from the glycerol backbone. This requires water, as the hydrolysis reaction re-introduces two hydrogen atoms and an oxygen atom -- a water molecule, in essence -- across the bond between glycerol and each fatty acid in a triglyceride molecule. The dependence of digestion upon hydrolysis is one of the reasons your digestive tract requires water to function.